As offshore drilling operations move into deeper waters, the hydrostatic pressure exerted on the wellbore by the column of mud in the marine riser may place excessive stress on relatively uncompacted formations, potentially causing the wellbore to fracture and lose circulation. Dual Gradient Drilling (“DGD”) refers to systems and methods of drilling in which the amount of pressure exerted on the wellbore by the hydrostatic pressure of the column of mud in the marine riser is reduced by a subsea pump system that assists in lifting the drilling returns from the well. In DGD operations, a heavier mud weight may be used to drill a wellbore resulting in a wellbore pressure profile that more closely mimics natural formation pressure trends. Advantageously, the use of heavier mud weights allows drilling operations to be conducted with substantially fewer casing strings, which are otherwise typically required to prevent wellbore collapse. However, the use of heavier mud weights makes it more difficult for drilling returns to reach the surface.
As such, a common objective of DGD is to reduce the hydrostatic pressure exerted on the wellbore by the column of mud in the marine riser to an amount equal to the seawater hydrostatic pressure on the seafloor. For example, in a drilling system using a 10,000 foot riser with 18.0 pounds per gallon (“ppg”) mud weight, the total hydrostatic pressure exerted on the wellbore by the column of mud in the marine riser is approximately equal to 0.52 (industry standard approximation value)*18.0 ppg*10,000 feet, which is 9,360 pounds per square inch (“psi”). However, the seawater hydrostatic pressure at 10,000 feet is approximately equal to 0.52*8.6 ppg*10,000 feet, which is 4,472 psi. As such, in DGD operations, a subsea pump system ideally provides lift that reduces the hydrostatic pressure exerted on the wellbore by the column of mud in the marine riser from 9,360 psi to 4,472 psi, thereby facilitating the flow of drilling returns to the surface.